It is often desirable to be able to ascertain a characteristic of a road surface that is beneath a moving vehicle, for example, whether the road surface is smooth or rough, pavement or gravel/sand, and hard-packed or loose-packed, and whether the vehicle is encountering a surface irregularity such as a pothole, broken pavement, or a railroad crossing, in order to modify certain vehicle operating parameters including, by way of example only, a target vehicle acceleration rate, a preferred automatic transmission gear selection and shift timing, and a projected/target braking (vehicle deceleration) rates.
Typically, a plurality of wheel speed sensors, often associated with the vehicle's antilock braking system (ABS), are employed to detect wheel slip at a driven wheel in response to the application of a braking torque to that wheel, as when actuating the wheel's associated brakes in either a vehicle braking mode or a traction control (controlled vehicle-accelerating applied wheel torque with simultaneous wheel braking) mode, and comparing the resulting change in detected wheel speed to the detected speed of other of the vehicle's wheels. However, in addition to likely requiring additional hardware in the form of wheel speed sensors at all four corners of the vehicle, such methods are necessarily limited to obtaining road surface characteristics during braking and traction-control events and, hence, do not provide any road surface information at any other time, as when the vehicle is otherwise operating at a relatively-steady speed or accelerating without engaging the traction control mode.